The present invention relates to an optical recording medium having, for example, a rewritable phase-change type recording layer. Particularly, it relates to an optical recording medium having a phase-change type recording layer excellent in storage stability in a non-recorded state and in a recorded state, an optical recording medium having excellent jitter properties in recording at a high transfer rate, and a method of recording/erasing on it.
On an optical recording medium having a phase-change type recording layer, recording/retrieving/erasing is carried out by utilizing a change in the reflectivity accompanying a reversible change of the crystalline state. Among such optical information recording media, a phase-change type optical disc (the phase-change type optical disc will sometimes be referred to simply as a disc in the present specification) is being developed and used practically as a low cost large capacity recording medium excellent in portability, weather resistance, impact resistance, etc. For example, rewritable CD such as CD-RW has already been used widely, and rewritable DVD such as DVD-RW, DVD+RW, DVD-RAM is being on sale.
Used practically at present as a means for recording on a rewritable phase-change type recording material, is to utilize a reversible change between the crystal phase and the amorphous phase to let the crystalline state in a non-recorded/erased state and to form amorphous marks at the time of recording. Usually, the recording layer is heated to a temperature higher than the melting point and then rapidly cooled to form amorphous marks, and the recording layer is heated and kept at a temperature in the vicinity of the crystallization temperature for a certain time to obtain a crystalline state. Namely, in general, a reversible change between the stable crystal phase and the amorphous phase is utilized. As a material for such a phase-change type recording layer, a chalcogen type alloy thin film is used in many cases. For example, a GeSbTe type, InSbTe type, GeSnTe type or AgInSbTe type alloy may be mentioned.
However, in proportion to increase in the volume of information in recent years, an optical recording medium with which recording/retrieving at a higher speed is possible, has been desired. Further, excellent storage stability of recorded information i.e. that information recorded on an optical recording medium does not deteriorate and is stable even after long-term storage, is also one of important performances required for an optical recording medium. The present invention has been made to achieve such a demand, and it is an object of the present invention to provide an optical recording medium with which recording/erasing at a higher speed is possible and which has a high storage stability of recorded signals.
Namely, in accordance with an aspect of the present invention, there is provided an optical recording medium comprising a substrate and a phase-change type recording layer which may have at least two phases provided on the substrate, wherein the phase-change type recording layer has a composition represented by the following formula (1) as the main component:
(AuxSb1xe2x88x92x)1xe2x88x92yGeyxe2x80x83xe2x80x83(1)
where x and y are numbers which satisfy 0.01xe2x89xa6xxe2x89xa60.4 and 0 less than yxe2x89xa60.3.
By using a recording layer having as the main component, a composition wherein a small amount of Ge is added to a Auxe2x80x94Sb type alloy rich in Sb, an optical recording medium having a phase-change speed higher than that of a conventional optical recording medium can be obtained, and accordingly recording/erasing at a higher speed can be carried out. The reason is considered as follows. Namely, it is estimated that in the Auxe2x80x94Sb type alloy, a quasi-stable crystal phase exists in addition to an amorphous phase and a stable crystal phase. In a conventional phase-change recording medium, a phase-change between the stable crystal phase and the amorphous phase with a high activation energy is mainly employed, and accordingly the speed of the phase-change is inadequate and recording/erasing cannot be carried out at a high speed. Whereas, in the present invention, by effectively utilizing the quasi-stable crystal phase of the Auxe2x80x94Sb type alloy, it becomes possible to employ a phase-change between the quasi-stable crystal phase and the amorphous phase with an activation energy smaller than the activation energy between the stable crystal phase and the amorphous phase for recording/erasing, whereby a higher phase-change speed can be achieved.
According to another aspect of the present invention, there is provided a method of recording/erasing on the above-mentioned optical recording medium having a recording layer having a predetermined Auxe2x80x94Sb type alloy composition as the main component in which the two different phases of said phase-change type recording layer are utilized as a recorded state and a non-recorded/erased state, respectively, wherein said phase-change type recording layer may have at least three phases of a phase A, a phase B and a phase C, and when the phase B is more stable than the phase A and the phase C is more stable than the phase B, said phase B is utilized as the recorded state or the non-recorded/erased state. In the present invention, it is defined that the phase B is more stable than the phase A when a phase change from the phase A to the phase B can take place at a lower temperature than a phase change from the phase B to the phase A.
According to another aspect of the present invention, there is provided a method of recording/erasing information signals on the above-mentioned optical recording medium having a recording layer having a predetermined Auxe2x80x94Sb type alloy composition as the main component, only at a reference clock period T of at most 15 nsec, wherein for recording, a high power laser pulse and a low power laser pulse are alternately irradiated, and said low power laser pulse includes a pulse with a pulse width of at least 0.9T.